The viscosity of diesel fuel increases with decreasing temperature. This is a cause of various cold-weather issues for diesel-engine systems in motor vehicles. In pumping cold, viscous fuel through a fuel system, an increased pressure drop occurs in various segments of the fuel system—especially across filters and through orifices. The increased pressure drop may prevent the fuel from being delivered to the engine at the required rate, causing start difficulties, stalls, rough idle and other performance issues. When the temperature drops to 5° C. below the ‘cold filter plug point’ (CFPP), some constituents of diesel fuel turn into a non-flowing gel, which may clog the fuel lines, filters, and orifices, and thereby prevent the engine from operating properly.
In many scenarios, accurate on-board diagnosis of excessive fuel viscosity, including gelation, may be valuable to a motor-vehicle operator, as various remedies are available to restore or protect engine performance in that event. Such remedies include moving the vehicle into a warm garage, purchasing fuel from a different source, or adding alternative fuel blends or other additives to the fuel system. However, excessive fuel viscosity may be difficult for the operator to diagnose, because its symptoms may be similar to those of a clogged fuel filter or a defective fuel pump or fuel injector, and may disappear as fuel temperature increases.
Moreover, the temperature alone is not a reliable indicator of diesel-fuel viscosity, which is affected by other factors including the moisture content of the fuel. More significantly, the blend of diesel fuel obtained from a service station may vary from region to region, and with the season. Lower-viscosity blends are distributed in cold climates during the winter months, for example, to partly address the issues noted above. Because of the variability of the fuel in a fuel tank at any given time, it is difficult to compute the viscosity of the fuel as a function of temperature, and thereby determine whether increased fuel viscosity could be the cause of a performance issue.
To address these issues and provide still other advantages, one embodiment of this disclosure provides a method for diagnosing a performance issue in a vehicle system having a diesel engine, a fuel system, and an on-board diagnostic system. The method includes cranking the engine, and, if a pressure sensed in the fuel system during or after the cranking differs from the pressure sensed during a previous operation of the engine, indicating excessive fuel viscosity in the on-board diagnostic system. In this manner, excessive fuel viscosity can be better distinguished from other fuel-system issues.
The summary above is provided to introduce a selected part of this disclosure in simplified form, not to identify key or essential features. The claimed subject matter, defined by the claims, is limited neither to the content of this summary nor to implementations that address the problems or disadvantages noted herein.